1. The third generation top quark was the last quark in the Standard Model to have its existence proven experimentally (in 1995). It is also the most massive of the quarks. Why was it so difficult to observe a top quark?

Creating massive particles requires a lot of energy. It took a long time to develop a particle accelerator which accelerated other particles to the energy required to create top quarks.

2. What observable phenomena does the Standard Model not explain?

Gravity, as well as mass, and the large number of constants in the Standard Model.

3. How much more massive is an up quark than an electron?

4. How many fermions are there in the Standard Model?

12 + 12 = 24 (remember the antiparticles)

5. The antiparticle of the electron (e-) is the positron. What is the charge and rest mass of a positron?